Zeta potential is an electric potential at the surface of shear relative to an electrically neutral solution far away and characterizes the apparent surface charge. Zeta potential may exist on a variety of surfaces including solids, such as particles or fibers, or planar surfaces. The term “surface”, as used herein, will include these and other examples on which a zeta potential may exist.
Knowing the zeta potential of macroscopic solid surfaces is useful. For example, the zeta potential of silicon can determine whether particles from solution are more or less likely to stick to it. If the silicon wafer has a surface charge and the particles in solution are charged with the same sign, then particles have a lower tendency to adhere. In chemical-mechanical planarization of silicon wafers, particles adsorb to the surface during polishing due to the opposite zeta potentials of the surface and the polishing particles. After polishing, increasing the pH such that the wafer and particles both have a negative zeta potential can cause the particles and wafer to repel one another. The zeta potential of the silicon wafer can be measured to ensure the desorption of polishing particles. See, for example, U.S. Pat. Nos. 6,736,992 and 6,676,766.
As another example, the biocompatibility of polymers used in medical devices depends on the polymer's zeta potential. Measurements of zeta characterize the adsorption of proteins or surfactants to solid surfaces. See, for example, Hiemenz, P. C.; Rajagopalan, R., Principles of Colloid and Surface Chemistry, Marcel Dekker Inc.: New York, 1997; Shaw, D. J., Introduction to Colloid and Surface Chemistry, Butterworths: London, 1986; Lyklema, J., Fundamentals of Interface and Colloid Science, Academic Press: London, 1995: Vol. II; Hunter, R. J., Zeta Potential in Colloid Science: Principles and Applications, Academic Press: London, 1981; Werner, C.; Koerber, H.; Zimmermann, R.; Dukhin, S.; Jacobasch, H. J., Journal of Colloid and Interface Science, 1998, 208, 329; Sides, P. J.; Hoggard, J. D., Langmuir 2004, 20, 11493-11498; U.S. Pat. No. 6,736,992; and U.S. Pat. No. 6,676,766.
A prior art method for determining the zeta potential of planar surfaces is based on flow in a thin gap between parallel plates. Two identical plate samples, or two different plates where the zeta potential of one plate is known, form the gap. Pressure driven flow through the thin-gap cell moves the charge in the diffuse layer on each plate, thereby producing convected ionic current. One measures either a streaming potential difference or a streaming current between the inlet and outlet of the thin-gap cell by means of two Ag/AgCl electrodes connected to an external meter having either a high or low impedance, respectively. The zeta potential is proportional to the measured streaming potential or streaming current and can be calculated therefrom with the aid of known relationships and parameters. This approach has been incorporated into a commercial product. For example, Anton-Paar, a company based in Graz, Austria, manufactures a scientific instrument based on this design for determining the zeta potential of planar solids.
Despite advances in this field, prior art methods and apparatuses for determining zeta potential of solid surfaces suffer some disadvantages. The prior art methods require two surfaces to form a thin channel; hence both surfaces must be equivalent or the operator must subtract out the contribution of a common surface. Furthermore, not all planar materials, such as thin fibrous mats, are easily formed into or attached to rigid planar surfaces in such a way that they can sustain a tight seal against the pressures required to cause flow through a narrow gap.
Not all of the prior art suffers from the problem of having two surfaces in close proximity, but there are other deficiencies. Scientists (see M. P. Sidorova, D. A. Fridrikhsberg, N. A. Kibirova Vestnik Leningradskogo Universiteta Vol. 2 121-123 (1973) and references cited therein) used a thin film of liquid flowing over a rotating disk. This reference is in Russian, but as Applicant understands it, the reference electrodes used to detect the streaming potential were affixed to the surface of the disk. The means of rotation requires the disk to admit a spindle through it and requires a moving electrical connection between a meter and the wires leading to the reference electrodes, which can introduce noise in the electrical signal. Other methods for determining zeta potential have been proposed. For example, two scientists tried to make a streaming potential measurement with a rotating surface in contact with bulk solution (see R. Knodler, A Kohling, and G. Walter, “Measuring Streaming Potentials on Flat Surfaces with Rotating Electrodes,” Electroanal. Chem. and Interf. Electrochem. 56 315-319 (1974). See also R. Knodler and D. Langbein Zeitschriftfur Physikalische Chemie Neue Folge. Bd. 98 S. 421434 (1975). See also R. Knodler and D. Langbein Zeitschrift fur Physikalische Chemie Neue Folge. Bd. 98 S. 421-434 (1975).). They made a ring of the desired material that was held between an axially concentric in-plane working electrode and an in-plane counter electrode. Their electrodes were integral to the disk and rotated with it. They measured the potential between two points (positions 2 and 3, in FIG. 1). Their method also required an aperture in the sample and also required a moving electrical connection. However, their method and experimental set-up failed to produce accurate results. In particular, they were not able to observe the 3/2 power dependence on the rotation rate expected on theoretical grounds and characteristic of this particular geometry. Experimenting with three different materials, they found exponents on the rotation rate of 0.27, 0.7, and 2.1. As a result, this new method fails to improve on the prior art.
Accordingly, there is a need by reason either of convenience or of feasibility for improved apparatuses and methods for determining zeta potential, particularly for apparatuses and methods for determining zeta potential of solid surfaces. Those and other advantages of the present invention will be described in more detail hereinbelow.